Mic Pre Scheme. Improvements? favorite op amp?

[chadwick]

Here is a seck mic pre and eq I'm copying. I wonder if the 500k trim pot is the best way to go here. Also I wonder if I have faced my pots the correct way, the small line being the users counter clockwise 'turn down' control.

What to do with the extra stage too, maybe bump up the signal or balance?

high shelf and low shelf look good? The mid sweep is very cool from 300 to 6k, sounds great to me too. Maybe I could add Q?

Like the 5532? I'm going to use a socket and post sound tests here.

Anybody want to chip in for some proto boards? Ten or so become much more reasonable then one or two.

Here it is....

http://www.achadwick.com/user/store/micpre_eq.pdf

[Rodgre]

I'm having a hard time checking out the schematic or the link to your site, but I wondered if this was the same pre out of the old Seck 18x8x2 board I used to use years ago at the first studio I worked at.

No one seems to mention those Secks and I wonder if it's because they're a little-known secret or because they didn't dig them. I recall kinda liking it.

Roger

[chadwick]

Thats the one! I have that in my studio and everytime I try to replace it I go running back. I even swapped all the op amps out one time and went back to the mix of 5532s and TL074s. I love it for drums, they seem very dynamic and punchy. I can't wait to plug it in and start swaping op amps in my circuit without having to disasemble an 18 channel console!.

The only thing is I've seen better looking front ends so I wanted to vet the scheme a little and see what comes up.

I'm doing this one because I'm familiar with it and like it. I would put the eq up against most. I'm sure there are other greater ones out there but this way I have a good starting point. Plust I'm using much higher quality components, all 1% metal film resistors and polypropolene WIMA caps.

I'm having a hell of a time finding verticle center detent pots in quanities less than 1000 and at values up to 500k. If anybod has better sources than Mouser or Digikey turn me on.

Try the link again my site has been acting funny so if its dead its my server. I don't know how they claim 99.... uptime when it seems down quite a bit.

Here is the link again just incase I didn't get it right...

http://www.achadwick.com/user/store/micpre_eq.pdf

an internet photo of the mixer.

[peterc]

Try Omeg for smaller quantities of pots

www.omeg.co.uk

Their min quantity is 25 & they are very reasonably priced. ( about $1-00 each for std items)

I have used them quite often, they are plastic conductive units & work very well. & you can order nice extras like PCB brackets for a small fee. Centre detents are also an option.

Speak to Margaret, their service is very good.

By the way, are the 2 resistors R16 & 17 supposed to be 6k8, not 1k8?

Looks like a very nice pre/EQ project, if I get some time I'll design a PCB for it.

Peter

[brianroth]

Interesting method for input gain adjustment, but the CMRR will likely vary all over the place as you adjust the pot.

The phantom resistors need to be 6k8, NOT 1k8 as drawn.

Use that second/unused 5532 opamp section to buffer the wiper of P3 to provide a low output Z.

Bri

[chadwick]

Ya I agree on the phantom power supply resistors but that is the value on the seck scheme. Here is the full repair manual.... I'll probably switch them for good measure anyway as suggested.

http://www.achadwick.com/user/store/Sec ... Manual.pdf

I like the buffer idea very much. How bout a balanced out put though? I think I need one more stage. Anybody? I know I've seen a scheme like that but the only ones I can find at the moment don't have a balanced output.

Also it does seem like a wierd trim gain thing.. I can only say that my mixer sounds really good and works great, hence me copying it. I hope to audition some fet op amps too. An op amp shoot out probably deserves its own topic. I wouldn't mind listening to the analog devices but I've probably grown fond of the 5532 bipolar

I'm waiting on the parts from mouser so I can finish my component outlines and really route it. This is the first route, just a trial run. All the parts were around 20bucks.



Anybody have a favorite xlr jack?? I want a simple plastic, no locking anything but with threaded screw holes that mount on the back side of the panel. Horizontal mount.

Does anybody have a scheme for a single LED [dual color] level/peak indicator??

[peterc]

I dont have a schem for a 2 dual LED but here is one you could adapt from 4 LED's down to 2.

http://1176neve.tripod.com/id12.html

I also have a balancing amp schem

http://1176neve.tripod.com/id6.html

If I can offer some advice for a PCB layout, it is to have the switches & pots mounted directly to the PCB. I have done both methods & the PCb mounted method makes assembly a lot quicker. It also helps with fault finding as it is really easy to solder a flying lead in the wrong place.

I have some pics here of the same preamp, one with PCB mounted comps & one with flying leads. I dont have any internal pics of the white faced unit, but it used the same PCB's as the dual, only mounted vertically.

http://1176neve.tripod.com/id15.html
http://1176neve.tripod.com/id28.html

Regards
Peter

[chadwick]

Yes that 4 LED thing would cost only pennies more and looks better. I totally agree on getting every component possible on the board. You only have to trouble shoot once and its all done. Mouser diagrams are incomplete and often inaccurate [alpha pots] so I'm waiting for the actual components to measure them and get accurate component outlines.

Do you have a higher res scheme of the balancing circuit..? Thanks for the LED Scheme! Beautiful work. I can see the effort and time that went in to it. This is easily the coolest audio forum on the web.

I also mock up most designs in 3d as well, time permitting. This helps for designing the chassis and getting proper fits visually, instead of always crunching the numbers.

[peterc]

Do you have a higher res scheme of the balancing circuit..?

Right click on the drawing & either "View" or "Save"

I love the 3D rendering, what programme do you use for it?

I use Traxmaker for PCB design, tried to do Kicad which has a render engine built into it but could not get it running properly.

Peter

[chadwick]

Thats just auto cad with Altium Protel exporting the traces and component outlines. That way I separately build the 3d Parts and populate the board. I actually ignore the data sheets after the build and use the chassis hole locations based on the 3d part placement. I often have to then export the metal part into solid works for our manufacturers.

I would be happy to make a couple DIY chassis for Tape op users, but every useful blank chassis is probably already available for much cheaper than a one off. Though a run of 100 could be very cheap.. Maybe the seck mic pre scheme, board, and mouser Bill of Materials could be a tape op project.